Homeless mothers with young children in their care contend with high rates of substance use, HIV risk, physical and mental health problems and parenting stress. These struggles are in addition to homelessness and meeting the basic needs of themselves and their children. However, a very limited number of studies have examined mother and child outcomes associated with housing and supportive services. Even with increased focus on those experiencing homelessness, the number of homeless families continues to rise, with the demand for temporary shelter so high that many cities are unable to meet the needs of these families. A comprehensive intervention that can be offered outside the shelter setting may offer greater reach to those experiencing homelessness who do not make it in to the shelter system, and for those communities that do not have shelters available. Among the young homeless, those under age 25, research documents that the majority (70%) have never used shelter services. Research attention towards identifying efficacious interventions for this population which address the multiple needs of these families is thus considered an important focus. The proposed intervention (Ecologically-Based Treatment, EBT) includes housing and supportive services and utilizes an ecological systems approach as the theoretical base. It was rigorously developed in a Stage 1 treatment development study with substance use disordered homeless mothers who were engaged through a crisis shelter. EBT showed several outcomes superior to shelter services and is therefore considered a good fit for a population who avoids the shelter but is in great need of housing and support services. Two hundred forty (N = 240) substance use disordered homeless young women between the ages of 18 to 24 years with a biological child under the age of 6 years in their care will be randomly assigned to one of three conditions: (1) housing and support services (EBT) + Treatment as Usual (TAU) (N = 80), (2) housing only (HO) + TAU (N = 80), or (3) TAU only (N=80). EBT includes 6 months of supportive services (case management, HIV prevention and the Community Reinforcement Approach) in addition to 3 months of rental assistance. HO includes 3 months of rental assistance, but without supportive services. TAU is usual services offered by a homeless youth drop-in center. Participants will be re-assessed at 3, 6, 9 and 12 months post-baseline. Theoretically derived mediators of change as well as a formal economic evaluation will offer important policy implications. Since homeless substance use disordered mothers and their children are at increased risk for a variety of adverse outcomes, the intervention may produce substantial health-care benefits to their families and society at large.